Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a scavenging portion and blowdown portion of exhaust gases. The timing of the first set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
However, the inventors herein have recognized potential issues with such systems. As one example, under certain operating conditions, such as high engine speeds, increased EGR may flow to the compressor, thereby increasing the compressor speed and temperature. Degradation to the compressor may occur if a gas temperature of gases entering the compressor and/or the speed of the compressor increases above threshold levels. The inventors herein have recognized that an EGR valve disposed in the EGR passage may be closed to reduce EGR flow to the compressor, thereby decreasing a temperature of exhaust gas flowing through and a speed of the compressor. However, the inventors have also recognized that closing the EGR valve may trap hot residual gases within the cylinders and/or first exhaust manifold and may also reduce blowthrough.
In one example, the issues described above may be addressed by a system for an engine comprising a first set of exhaust valves fluidly coupled to an exhaust passage, upstream of a turbocharger turbine, a first emission control device (ECD), and a second ECD disposed within the exhaust passage, the second ECD positioned downstream of the first ECD; and a second set of exhaust valves fluidly coupled to an intake passage and the exhaust passage, between the first ECD and the second ECD. In this way, exhaust gases expelled from engine cylinders via the second set of exhaust valves may be selectively routed to the exhaust passage, instead of the intake passage, thereby reducing a flow of hot exhaust gases to a turbocharger compressor while still enabling a blowthrough function.
As one example, the second set of exhaust valves may be fluidly coupled to the intake passage, upstream of a turbocharger compressor driven by the turbocharger turbine, via an exhaust gas recirculation (EGR) passage and fluidly coupled to the exhaust passage via a flow passage, the flow passage coupled to the exhaust passage, downstream of the turbocharger turbine and between the first and second ECDs. One or more valves may be disposed within the flow passage and the EGR passage. Thus, by adjusting a position of the one or more valves, exhaust gases from the second set of exhaust valves may be routed to the exhaust passage, downstream of the turbocharger turbine and between the first and second ECDs. This may reduce a flow of exhaust to the turbocharger compressor while still allowing exhaust and blowthrough air to flow from the second set of exhaust valves to the exhaust passage. Further, by fluidly coupling the second set of exhaust valves to the exhaust passage, between the first and second ECDs and downstream of the turbine, a speed of the turbocharger may be reduced (thereby reducing the compressor speed) and oxidation of the first ECD (e.g., if the first ECD includes one or more catalysts) via blowthrough air from the second set of exhaust valves may be reduced, thereby reducing a temperature increase of the first ECD. In this way, degradation of the compressor may be reduced while maintaining the blowthrough function and reducing degradation of the first ECD.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.